1. Field of the Invention
The present invention relates to a thermal printing method and a thermal printer, and more particularly to a method of printing characters and half-tone images by using a different dot print pattern from each other, and a thermal printer for use with the method as well.
2. Related Art
Thermal printers use a recording head having M recording elements (M is an integer) disposed in a main scan direction to simultaneously record M dots on a recording paper while moving the recording head or the recording paper in a sub scan direction perpendicular to the main scan direction.
Recently, in order to provide a half-tone image, an area gradation method has been proposed, for example, in a commonly assigned U.S. Pat. No. 5,297,878. According to this method, one pixel is divided into N sub-lines (N is an integer), and a half-tone is produced by changing the number of the sub-lines where a dot is recorded within a pixel. For this method, such-a recording head is used that has heating elements with a length much less than the length of one pixel in the sub scan direction. Each time the recording head or the recording paper is moved by one sub-line amount in the sub scan direction, the heating elements are driven to record dots on the sub-lines designated by print data. Thereby, the size or the area of a dot in each pixel is changed in N steps to reproduce the gradation.
This area gradation method is effective for a printer which cannot control the density of a dot itself, such as a thermal wax transfer type printer which heats the back of an ink film and transfers melted or softened ink onto the recording paper.
Besides half-tone images, binary images such as characters and lines or an image containing a half-tone image in combination with characters are often required to be printed. Generally, a character is constituted of I.times.J pixels (I, J is an integer) and recorded in the form of a dot pattern wherein each dot is recorded in the whole area of the corresponding pixel.
However, in a conventional thermal printer using the above-described area gradation method, the same area gradation printing sequence is applied to the character printing as to the half-tone image printing. As a result, the characters may be chipped on the edge or may have an uneven density depending on the originals. Moreover, because the heating elements are activated at each sub-line position even for the character printing, the efficiency or the printing speed is lowered compared with a binary dot printer.